1. Field of the Art
This invention relates to a process for producing dimethylamine by the gas phase catalytic reaction of methanol and ammonia. More specifically, the present invention relates to a process for producing dimethylamine having a specific feature in the catalyst employed.
Dimethylamine is an important chemical intermediate as the starting material for various solvents, pharmaceuticals, organic rubbers, surfactants, dyeing aids, etc., and is generally produced by reacting methanol with ammonia in gas phase at an elevated temperature (around 400.degree. C.) in the presence of a solid acid catalyst (hereinafter called the conventional catalyst) having dehydration and amination action such as .gamma.-alumina, silica, silica-alumina and thoria. In this reaction, other than dimethylamine (hereinafter abbreviated DMA), monomethylamine (hereinafter abbreviated MMA) and trimethylamine (hereinafter abbreviated TMA) are also produced almost inevitably, and these by-product amines, for which demand is less than that for DMA, are separated from the reaction product and then transferred to the reaction system for reutilization.
Dimethylamine is separated from the reaction product of methylamines by distillation. However, since TMA forms complicated azeotropic mixtures with ammonia, MMA and DMA, very cumbersome and large scale distillation operations are required, whereby the energy consumption in the DMA recovery process becomes very great. Examples of the recovery process are shown in detail in, for example, "Revised Complete Collection of Manufacturing Flow Chart" (published by Kagaku Kogyo Sha Co., Ltd., Apr. 25, 1978).
For realization of reduction in production cost of DMA and in the size of the device, it is critically important to suppress formation of the methylamines other than DMA (MMA, TMA), particularly TMA, to a minimum, thereby promoting formation of DMA. However, the final proportion of the three kinds of methylamines formed in governed by thermodynamic equilibrium, and the proportion of MMA and DMA formed will be higher as the temperature becomes higher, and the ratio N/C of the number of nitrogen atoms to the number of carbon atoms in the reaction mixture becomes higher, with the proportion of TMA becoming smaller. For example, when the reaction temperature is 400.degree. C., and the ratio of ammonia to methanol is 1:1 (weight ratio), the equilibrium proportions of the respective amines formed, calculated thermodynamically, are 0.284 for MMA, 0.280 for DMA and 0.436 for TMA.
In the case where the above conventional catalyst is used, the MMA formation reaction or TMA formation reaction is relatively rapid, and therefore the proportion of DMA formed in the three kinds of methylamines throughout the entire reaction region will never surpass this equilibrium value. Thus, large amounts of MMA and TMA must always be recycled together with unreacted ammonia to the reaction system.
Various methods have been known for promotion or suppression of a specific amine among the three kinds of methylamines. For example, by varying the reaction conditions, the level of equilibrium itself can be shifted to control the yield in favor of a specific amine. Generally speaking, as the reaction temperature and the ratio (N/C) of the number of nitrogen atoms to the number of carbon atoms become higher, MMA and DMA will be more advantageously formed. However, as shown in Table 1 set forth hereinafter, the change in the DMA formation ratio at the equilibrium does not greatly depend on the change in the reaction temperature and N/C. At higher reaction temperatures, the amounts of impurities produced such as carbon dioxide, methane, formaldehyde, higher amines, etc. are increased. On the other hand, at higher ratios N/C, the amount of ammonia to be circulated is increased, resulting in enlargement of the apparatus. For the reasons given above, it is not recommended to use reaction conditions outside those generally employed, namely, a reaction temperature of 360.degree. C. to 450.degree. C. and N/C of 1.2 to 3.0.
2. Prior Art
The method of promoting formation of DMA by modifying chemically the conventional catalyst such as silica-alumina has been proposed. For example, Japanese Patent Publication No. 486/1970 discloses a method for improving the yield of DMA by the use of a catalyst based on the silica-alumina impregnated with a sulfide such as that of Re, Ag or Co.
In recent years, as the catalyst for producing a specific methylamine (e.g., MMA or DMA) with high selectivity, various zeolites are coming to the fore of interest. Among them, mordenite type zeolites are also included. For example, Japanese Laid-Open Patent Publication No. 113747/1981 discloses a method for obtaining selectively MMA from ammonia and methanol with the use of various zeolites inclusive of mordenite. Also, Japanese Laid-Open Patent Publication No. 46846/1981 discloses a method for producing DMA from MMA with the use of the same catalyst as mentioned above. Japanese Laid-Open Patent Publication Nos. 148708/1979 and 104234/1980 disclose the method for promoting formation of primary and secondary amines from alcohol and ammonia by the use of the synthetic zeolite FU-1 produced from materials containing a quaternary ammonium salt. U.S. Pat. No. 4,082,805 discloses that primary and secondary amines are obtained preferentially from alcohol and ammonia by the use of the synthetic zeolite ZSM-5 and others.
In any of the methods employing such a zeolite as the catalyst, the proportions of MMA and DMA formed surpass the thermodynamic equilibrium values. This is probably due to the effect of the so-called molecular shape selectivity, resulting from selective blocking of molecules sterically expanded (TMA) at the fine pore inlets, since the sizes of the fine pores within the crystalline structure of zeolite are at the level of molecular sizes.
Zeolites exhibiting shape selectivity for the reaction to form methylamines from ammonia and methanol known in the art are inclusive of mordenite, erionite, clinoptilolite, zeolite A and other special synthetic zeolites. Among them, particularly, mordenite is disclosed to have a marked effect of suppressing formation of TMA in Japanese Laid-open Patent Publication No. 169444/1982.
Mordenite is a crystalline aluminosilicate represented by a formula Me.sub.1/n.(AlSi.sub.5 O.sub.12).3H.sub.2 O (where Me is a n-valent metal atom, hydrogen atom, etc.). By the use of mordenite for synthesis of methylamines, the selectivity of TMA is reduced to a great extent, and the selectivity of MMA or DMA is increased. However, mordenite is liable to form coke, and its catalytic properties are very susceptible to influence by coke deposition due to its crystalline structure. For this reason, the synthesis temperature (around 400.degree. C.) poses a problem in the aspect of the catalyst life, and practically it is necessary for prevention of coke formation to carry out the reaction at a temperature not higher than 360.degree. C., preferably not higher than 340.degree. C. Accordingly, a necessary condition for practical use of mordenite is that it has a sufficiently high catalyst activity at such a low temperature.
Me of mordenite is exchangeable with cations such as those of various metals, hydrogen and ammonia, and, depending on these cations and their amounts, the fine pore size and the acidic nature on the fine pore surfaces within the crystalline structure or the acidic nature on the fine pore surfaces based on interstices between the primary particles is influenced, whereby the catalyst activity and the selectivities of the amines vary greatly.